With the rapid growth of network capacity and vast increase of network complexity, network configuration is getting more complicated. Thus the DHCP (Dynamic Host Configuration Protocol) based on the BOOTP (Bootstrap Protocol) has come into being. With a large expansion, it has been widely used. The DHCP can dynamically assign a duration-limited IP address to an access subscriber terminal. When the duration is ended or the subscriber terminal definitely shows that it would discard the IP address, the IP address can be used by other subscriber terminals. In this way, the efficiency of resource utilization is raised. The dynamic allocation can be used when assigning an IP address to a temporary access subscriber terminal or a group of subscriber terminals that share a group of limited IP addresses and need not permanent IP address. While a new access subscriber terminal needs a permanent access to a network, but the network IP address is very limited, in order to recover the IP address when the subscriber need not permanently access to the network again in the future, the dynamic allocation can also be used.
FIG. 1 shows an actual network application environment of the present invention. The network comprises a 3-level exchange with DHCP relay functionality, a main DHCP server and a backup DHCP server in one segment of the network connecting to the exchange, and DHCP clients, i.e., subscriber terminals, in another segment of the network.
In DHCP, the dynamic IP address allocation is performed through the exchange of DHCP packets between the DHCP server and subscriber terminals. The DHCP packet is a broadcast packet, which cannot cross into another segment of the network, so a DHCP server can only serve subscriber terminals in its own segment. Owing to resource limitation, it is impossible to configure a DHCP server for each network segment, and a DHCP server is usually set at a stand-alone segment for security reason. Therefore, it is necessary for a DHCP server to serve subscriber terminals in other network segments. The relay function of DHCP makes DHCP broadcast packets re-transmitted between segments of a network. By this means, a DHCP server can serve subscriber terminals in different segments.
In a network running DHCP, some subscriber terminals get IP addresses without through DHCP server but through illegally occupying them assigned to other clients. At present, the problem of IP address cheating is dealt with as follow: during bootstrap, the computer system of a subscriber terminal sends out a charge-free ARP (Address Resolution Protocol) packet to check whether its IP address has been occupied; if so, the computer system will send out an address contention report. While this method cannot solve the problem of IP address cheating root and branch, an IP address cheater may still get access to network.